1. Field of the Invention
The present invention relates generally to the dissolution of metals in an aqueous bath containing hydrogen peroxide and sulfuric acid, and more particularly, relates to an improved process of etching copper in printed circuit board processing and the like.
2. Prior Art
In the manufacture of printed circuit boards, a laminate of copper and etch-resistant material, usually plastic, is used. Holes are drilled at appropriate locations on the board and the walls of the holes are metallized such as with copper to provide electrical contact between the two faces or surfaces of the board. Methods of metallization are known and include the steps of cleaning, catalyzing and electroless copper deposition.
The electroless plating or deposition of copper over the existing copper clad on the boards requires etching of the copper cladding to some degree in order to secure adhesion of the electroless copper plate on the copper clad. Two kinds of etchants are typically used commercially for this process:
(a) those based on acidic persulfate solutions; and PA1 (b) those based on hydrogen peroxide-sulfuric acid mixtures.
In the last few years, etchants based on hydrogen peroxide-sulfuric acid mixtures have become more popular because they are more economical and can be operated for a long period of time by adding acid and peroxide to regenerate the etchant. Further, waste treatment for removal of copper is not necessary since copper values can be recovered from the spent etchant with relative ease. For instance, as disclosed in U.S. Pat. No. 4,144,119, the bath or etchant is allowed to cool to room temperature to induce precipitation of copper compounds which may then be recovered by filtration. Other examples of hydrogen peroxide-sulfuric acid etchants are disclosed in U.S. Pat. Nos. 3,293,093, 4,130,455 and 4,174,253. Hydrogen peroxide-sulfuric acid etchants are usually operated at 110.degree.-130.degree. F. and when the etchant becomes saturated or approaches saturation with copper, or some other condition such as substantially decreased etch rate, etc. exists, the etchant is pumped into a holding or crystallization tank where it is kept at room temperature for several hours to crystallize out, i.e., precipitate, copper sulfate, usually as copper sulfate pentahydrate (CuSO.sub.4.5H.sub.2 O), dissolved during operation. Following crystallization or precipitation of the copper, the etchant is put back into operation after proper adjustment of the levels of hydrogen peroxide, sulfuric acid and other stabilizers, defoamers, etc. Examples of suitable stabilizers, defoamers, inhibitors and the like which may be used are disclosed in U.S. Pat. Nos. 3,293,093, 4,110,237, 4,130,455, 4,140,646, 4,144,119, 4,158,592, 4,158,593 and 4,174,253, and the disclosures of these patents are incorporated herein by reference. Any one or some suitable, desired combination of these additives may be provided in the etchant, depending upon the desired function.
The amount of copper which may be removed during crystallization is limited by the difference between solubility of the copper in the aqueous hydrogen peroxide-sulfuric acid etchant at the operating temperature (110.degree.-130.degree. F.), and the solubility at the precipitation temperature (generally room temperature, or about 70.degree. F.). Normally, the concentration of sulfuric acid in the etchant is maintained at a substantially constant level during use of the etchant, as for example about 14 or 15% by volume, and the etchant is operated until it approaches saturation. The etchant is then removed from operation and pumped into a holding tank, as noted previously, to precipitate out the copper sulfate by cooling of the solution. Usually, economic considerations dictate that the cooling of the solution is accomplished simply by allowing the solution to stand at ambient conditions. Refrigeration or other active cooling steps to precipitate out additional copper, then reheating to operating temperature, are usually not economically practical. In accordance with the prior art methods, a quantity of dissolved copper metal will precipitate out of the solution, and thereafter, the hydrogen peroxide, sulfuric acid and any other additives are replenished and the solution put back into operation.
The difficulty with the prior art methods discussed above is that the etchant must be frequently removed from operation at relatively short intervals to precipitate out the copper. This results in substantial down time for the etchant and increased handling and cost.